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A knee replacement is an operation to replace damaged parts of the knee joint. It can be either a total knee replacement (TKR) or a partial (unicompartmental) knee replacement. The new part is called a prosthesis.

Surgery to replace a worn-out knee joint is very common. It is increasingly popular, as the outcomes have become better and better over the last decade or so. John Cleese says he's "practically bionic now" having had his knee replaced, as well as both of his hips.

When a knee replacement is needed

The usual reason that someone has a knee replacement is because they have very painful arthritis in their knee.

You should always bear in mind that a knee replacement is a major operation and you should really only be considering it when you have run out of other options. A doctor can tell you that you have arthritis in your knee and they can tell you that you could have a knee replacement but only you can decide if the time is right for you. Most people who decide to have a knee replacement are already taking painkillers every day but are still not able to walk far and need to use a stick.

Looking at all of the research on knee replacements (and it's good to know that there is lots), it would seem that the people who do best after a knee replacement are the ones with severe arthritis but not so bad that the joint is completely destroyed. This could be because it's really important to have strong muscles around the knee in order to make the best recovery and people who have the most advanced disease tend to have very weak leg muscles.

The main reason for needing a knee replacement operation are pain in the knee - a knee replacement operation is essentially a painkilling operation. The pain can affect how far you can walk and may affect your ability to work. It is usually at its worst when you stand on the affected leg and is often really bad at night. You won't necessarily need a knee replacement if you have been told you have arthritis in your knee, as there are lots of other treatments that will help if the symptoms aren't severe. However, if the pain is severe despite taking painkillers, losing weight and physiotherapy, and you are finding that you are increasingly disabled by it, a knee replacement operation may be a sensible option.

Symptoms will often vary from day to day for no apparent reason. This is really common. Some people think their symptoms vary according to the weather or according to how much they have been doing - but it can be completely random.

Sometimes you will be aware of a grating or grinding feeling coming from your knee. This is called crepitus. On its own this does not necessarily indicate a serious problem with your knee.

The main reason for needing to have a knee replacement is arthritis in your knee:

Osteoarthritis

Osteoarthritis (OA) of the knee is the most common reason for a knee replacement. It can be primary or secondary:

Primary osteoarthritis:

Is not caused by previous damage or injury to the joint.

It is more common in people who have a close relative, particularly a brother or sister, with osteoarthritis.

Using a walking stick or cane - and in the UK an umbrella! Use the walking aid on the side opposite to your affected (or worst) leg. For example, if you have a bad right knee, hold the walking aid in your left hand. Then move the bad leg and the aid at the same time, so that the load is shared.

Tests before knee replacement surgery

About six weeks or so before your operation you will have an appointment for a 'pre-admission' or 'pre-assessment' clinic. At this clinic a nurse will assess your fitness for your knee surgery.

There are several tests that may be needed and they include:

Blood tests - to check that you aren't anaemic and that your kidneys and liver are working well enough for you to undergo the operation.

Urine test - to make sure you haven't got a urine infection and that there isn't any glucose in your urine.

Blood pressure.

Infection screen - this includes looking for meticillin-resistant Staphylococcus aureus (MRSA). MRSA is a germ (bacterium) that is difficult to treat and can cause complications of a knee replacement.

A heart tracing (electrocardiogram, or ECG).

You may have the chance to speak to an anaesthetist, physiotherapist, occupational therapist or social worker at this clinic but this isn't always possible.

Risks and benefits

Make sure that at some point before your operation, you have the opportunity to discuss all the potential risks of the surgery for you. This should be clear and in plain language that you understand fully. If you have other medical problems, such as heart disease, diabetes or a tendency to deep vein thrombosis or if you are obese, you should also have explained to you how these things may increase the risks of the operation for you.

What type of anaesthetic will I need?

At the pre-assessment clinic you can talk about the type of anaesthetic for your knee replacement. An anaesthetist will explain to you which type of anaesthetic is most suitable for you but your preference will always be taken into account. Most people have a spinal anaesthetic.

Care after the operation

Please give some thought as to how you will be looked after once you have had the operation, well in advance. Most people like to be independant, but you are going to need support with day-to-day activities for a while. If you have an able-bodied partner, this might fall to them, but otherwise you may need a friend or relative to come and stay with you for a while. Some people may arrange to stay in a care home until they have their mobility and independence back.

Knee replacement operation

The operation usually takes between 1 and 2 hours. The surgeon will make a cut down the front of your knee and then cut away the damaged surfaces of the ends of the thigh bone (femur) and shin bone (tibia) along with a little bit of the underlying bone. The two surfaces that have been removed are then replaced with specially shaped artificial surfaces. The new surface that covers the top of the shin bone (tibia) is usually made of metal and plastic. Sometimes it is only made of metal and a separate piece of plastic is inserted; this is called a mobile-bearing knee replacement. The plastic, whether separate or part of the covering of the shin bone (tibia), allows the two ends of the bones to glide over each other smoothly. Your knee cap (patella) may also be given a new surface, although sometimes it's left alone.

Some surgeons are using minimally invasive techniques - sometimes called keyhole surgery. This means that they make just one or two very small cuts instead of one long cut and use specially designed surgical instruments and telescopes. Your surgeon will discuss with you if this is available.

You will be able to go home once you are eating and drinking normally and are mobile enough to be safe where you are going after you leave hospital.

Types of knee replacement surgery

Knee replacements can be divided into two types:

Total knee replacement (total knee arthroplasty):

Most knee replacement operations involve replacing the surface of the bottom end of your thigh bone (femur) and the upper surface of your shin bone (tibia)

A total knee replacement may also involve replacing your knee cap (patella) with a dome-shaped plastic one.

Unicompartmental (partial) knee replacement:

If your arthritis only affects one side of your knee (usually the inner side) you may be offered a partial knee replacement.

A partial knee replacement involves less of your knee being operated on and the recovery is usually quicker.

It is more likely that a partial knee replacement can be done using minimally invasive techniques.

Whether total or partial, the replacement parts are made of a combination of metal and plastic; the metal parts replace the surfaces of the thigh bone (femur) and shin bone (tibia) and the plastic replaces the meniscus or menisci. (See 'causes' section for more information about the anatomy of the knee joint).

The metal parts may be fixed in place using special cement (cemented) or they may not be fixed (uncemented) but designed so that the your bone grows over them and fixes them in place that way.

Complex or revision knee replacement

This may be needed if arthritis has damaged more than the usual amount of bone or when a previous knee replacement has to be re-done (revised). Sometimes, in very complex situations such as following surgery for bone cancer, the components will be designed specifically to fit in your knee.

Which type should I have?

Your surgeon will discuss this with you. It will depend on how much of your knee is affected by arthritis - it may not be possible to know this until your surgeon has started your operation.

If you have a partial knee replacement it is more likely that you will need to have it done again, than if you have a total knee replacement (TKR). Sometimes the reason for choosing to have a partial knee replacement is that it leaves the option to have a TKR at a later date. However it's also more likely that you will need to have your total knee replacement re-done, if you had a partial knee replacement done before having your total knee replacement.

There are over 150 different designs of knee replacement and some of the differences between all of the different types and makes of knee replacement parts aren't known, particularly how they perform in the long term. In many countries, registries have been set up so that anyone who has had a knee replacement is entered into the register. The information collected is used to monitor how their replacement is performing. In the UK patients also enter information about their health and quality of life before and after their operation.

Recovering from knee replacement surgery

For the majority of people knee replacements are very successful. There is a lot of evidence from research showing that patients have less pain and are much more mobile after surgery and this often greatly improves their quality of life. Outcomes are getting better too, as more research is carried out on what the best operation is and how to reduce the risk of complications.

However about 8 people out of 100 are unhappy with their knee replacement 2-17 years later. If they have had to have their knee replaced a second time (revised), they are twice as likely to be unhappy with the outcome.

Will I need to be seen again after my operation?

Within about 8 weeks of your operation, you will be followed up by the hospital where you had your surgery. You will usually be offered further follow-up appointments.

Possible complications of knee replacement surgery

Bleeding

Blood transfusion may be needed.

Pain & stiffness

Pain can be reduced by different anaesthetic techniques used at the time of your operation.

It is important to make sure that you get adequate pain relief. You need to be able to move about and then start to walk as soon as you are able after your operation.

Venous thromboembolism

All patients are given thromboprophylaxis (medication, foot pumps, below knee stockings) - unless it would be dangerous to do so. (Thromboprophylaxis is the name for anything that reduces the chance of getting a venous thromboembolism).

This reduces the chance of suffering from the most severe but rare form of thromboembolism, which is a pulmonary embolism (PE). It reduces the risk of dying from a PE by 70%.

If you have already had a venous thromboembolism before or are closely related to someone who has, this makes it more likely that you will suffer from one when you have your knee surgery. Cancer and chemotherapy, as well as being obese, also increase your risk of this complication.

Nerve damage

It is common to have a numb area of skin to the outer side of the operation scar.

This may improve over 2 years but doesn't always recover completely.

Occasionally a particular nerve, called the common peroneal nerve, is damaged during a knee replacement.

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